1. Field of the Invention
This invention relates to a method and apparatus for repairing defects in photomasks usable in the manufacture of semiconductor microcircuits and other electronic components to transfer a circuit pattern onto a workpiece and, in particular, to removing defects which are connected to and protrude transversely from the photomask pattern. The defects are a form of a defect commonly termed black dot defects.
2. Description of Related Art
In the manufacture of circuit patterns on electronic components such as the manufacture of integrated circuits on semiconductor substrates photomasks are used to transfer the desired circuit pattern onto the substrate workpiece. Typically, a photomask comprises a patterned metal film such as chromium, nickel or aluminum in a thickness of about 1,000 .ANG. deposited on a transparent base such as glass or quartz. The photomask is generally manufactured by depositing a thin film of the metal on the surface of the transparent substrate, coating the film metal with a photoresist coating, exposing a pattern on the photoresist coating, developing the resist coating, and removing the metal from the unprotected areas of the film by etching leaving patterned metal film on the substrate.
The pattern contained in the photomask is reproduced onto the surface of a workpiece typically by placing the over the workpiece and irradiating a radiation-sensitive resist material on the workpiece. The variety of radiation sources for lithography that have been used or proposed include visible light, ultraviolet light, x-ray radiation, electrons and ions. When illuminated by the radiation, the metal pattern on the photomask serves to selectively block portions of the radiation beam while allowing other portions to be transmitted therethrough. In this manner, very complex geometries having very narrow line widths can be reproduced allowing the economical production of very large scale integrated circuits and other devices.
A photomask is typically employed a large number of times for the production of numerous electronic devices. This places stringent demands on the quality of a photomask since any flaws or defects are reproduced in the workpiece which directly effects the operability of the workpiece.
The process of fabricating photomasks, however, typically causes several defects in the pattern. The defects are usually classified as opaque and clear with opaque defects arising where excess metal (chrome) exists in an unwanted area and clear defects arising where metal (chrome) is missing from a desired area. Opaque defects in a mask are commonly termed black dot defects and typical black dot defects are shown in FIG. 1. Currently it is possible to repair such black dot defects (i.e., remove excess chrome) by laser vaporization. The repair of isolated black dot defects shown as 13d in FIG. 1 and clear defects are not the subject of this application. This invention is directed to black dot defects which are connected to the pattern such as 13a, 13b and 13c of FIG. 1 and the following description will be directed to this type black dot defect.
Method and apparatus are commercially available for repairing black dot mask defects by focusing laser light energy on the defect to vaporize and scatter the metal film molecules. Such apparatus using microscope optics has become automated including computer control of an optical X-Y table and the laser source so that a mask can be scanned, the position of opaque defects noted and stored as compared to a proper mask pattern and the data placed in the apparatus for computer controlled positioning of the mask and operation of the laser to ablate (vaporize) the defects.
There are certain problems, however, in using a laser or other form of energy to ablate a black dot defect. For example, the ablated chrome could land on top of the adjacent chrome image being repaired and become an annealed chrome layer due to the heat of the ablated chrome. After repair, for example, the chrome thickness next to the repair can be up to twice as thick as the original chrome pattern. If an additional laser ablation is required, the annealed material would not be readily ablated due to its hardened annealed characteristics. A further problem is the pattern edge of the repair cannot be well defined because heat is transferred to the chrome line. This can make edge reconstruction of fine features difficult, particularly in tight geometries having narrow line widths of less than about 1 micron. Heat supplied to remove the black dot defect can also thermally damage the pattern line. Processes which melt or evaporate the attached black dot defect, therefore, create a thermal/mechanical situation where repair process tooling needs to be optimally set to effect a satisfactory black dot repair.
There are a variety of photomasks including, x-ray masks, which typically have an order of magnitude of density greater than optical masks and laser repairs of optical mask defects while difficult is generally easier than repairing x-ray masks. In general, the x-ray masks have a larger metal pattern thickness which results in an increased thermal effect due to the laser ablation process.
A number of patents have issued in the are of repairing defective photomasks and include U.S. Pat. Nos. 4,200,668; 4,548,883; 4,727,234 and 4,933,565. These patents discuss the various type of photomasks and the different methods employed to repair black dot defects on the mask and the disclosures of each of the above patents are hereby incorporated by reference. The following description will be directed to the repair of chrome metal masks but it will be appreciated by those skilled in the art that the invention applies to other masks such as optical masks, x-ray masks, attenuated phase shift masks and alternating phase shift masks which may be made using a variety of metals as the patterning design such as chrome, MoSi, and chrome oxide fluoride.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the present invention to provide a method to repair black dot defects connected to the patterned circuitry in photomasks used to make electronic components such as semiconductors.
It is a further object of the invention to provide an apparatus to repair black dot defects connected to the patterned circuitry in photomasks used to make semiconductor electronic components.
It is another object of the invention to provide photomasks made in accordance with the method and apparatus of the invention.
Other advantages of the invention will in be readily apparent from the following description.